The simplest solution is, after the marriage has taken place, is to go to the Italian Immigration office (or local Anagrafe) and apply for an Article 10 residence card (Family member of a Union citizen). Italy, as apposed to some other EU countries, will issue a Article 10 card to the spouse of an Italian citizen.
DIRECTIVE 2004/38/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 29 April 2004
on the right of citizens of the Union and their family members to move and reside freely within the territory of the Member States amending Regulation (EEC) No 1612/68 and repealing Directives 64/221/EEC, 68/360/EEC, 72/194/EEC, 73/148/EEC, 75/34/EEC, 75/35/EEC, 90/364/EEC, 90/365/EEC and 93/96/EEC
Article 1
Subject
This Directive lays down:
(a) the conditions governing the exercise of the right of free movement and residence within the territory of the Member States by Union citizens and their family members;
(b) the right of permanent residence in the territory of the Member States for Union citizens and their family members;
(c) the limits placed on the rights set out in (a) and (b) on grounds of public policy, public security or public health.
Article 2
Definitions
For the purposes of this Directive:
- ‘Union citizen’ means any person having the nationality of a Member State;
- ‘family member’ means:
(a) the spouse;
(b) the partner with whom the Union citizen has contracted a registered partnership, on the basis of the legislation of a Member State, if the legislation of the host Member State treats registered partnerships as equivalent to marriage and in accordance with the conditions laid down in the relevant legislation of the host Member State;
(c) the direct descendants who are under the age of 21 or are dependants and those of the spouse or partner as defined in point (b);
(d) the dependent direct relatives in the ascending line and those of the spouse or partner as defined in point (b);
...
Contrary to opinions expressed elsewere, a Brazilian citizen, who is a family member of an Union citizen, will have certain rights that other Brazilian citizens, who are not a family member of an Union citizen, will not have.
Article 9
Administrative formalities for family members who are not nationals of a Member State
- Member States shall issue a residence card to family members of a Union citizen who are not nationals of a Member State, where the planned period of residence is for more than three months.
...
Article 10
Issue of residence cards
- The right of residence of family members of a Union citizen who are not nationals of a Member State shall be evidenced by the issuing of a document called ‘Residence card of a family member of a Union citizen’ no later than six months from the date on which they submit the application. A certificate of application for the residence card shall be issued immediately.
Once this card (or certificate) has been issued, the documented right of residence in Italy exists and therefore there will be no problems when leaving or entering the Schengen Area.
Any possible fine, due to a previous overstay, would be delt with when the application for the Article 10 residence card is being made. Since the marriage at this point has taken place (and thus the right of residence allready exists), the officials may come to the conclusion that it is not worth the effort (but with bureaucracies you never know).
With the Article 10 residence card the spouse can also work, so this card will be needed anyway sometime in the future so you might as well get it done now.